Bandgap voltage references are circuits that generate a temperature-stable voltage by combining a p-n junction voltage with a thermal voltage. In many circuits and devices (e.g., analog-to-digital converters, etc.), a precise voltage reference is required to operate the circuits and/or devices at a precise level. Persons of skill in the art will readily appreciate that temperature affects a threshold voltage at which a transistor operates. Generally, a bandgap reference is used to generate such a reference voltage that is temperature independent. To form a bandgap reference, a complementary-to-absolute-temperature (CTAT) voltage reference is generated that decreases with increasing temperature (i.e., the CTAT voltage has a negative temperature coefficient). The bandgap reference also forms a proportional-to-absolute-temperature (PTAT) voltage that increases with increasing temperature (i.e., the PTAT voltage has a positive temperature coefficient). When the PTAT and CTAT voltages are combined properly, their respective temperature coefficients cancel each other out, thereby resulting in a temperature stable voltage. In other examples, a PTAT voltage is also generated for other purposes (e.g., to provide a voltage that varies and represents temperature, etc.).
FIG. 1 illustrates a known fully isolated NPN-based bandgap reference circuit 100 including a PTAT voltage reference generator. Generally, in a fully isolated circuit, the only nodes that are coupled with the substrate are solid nodes (e.g., ground, voltage supply, etc.), thereby preventing collecting charge carriers from being injected into the example circuit 100 by other circuits. To isolate the example circuit 100 of FIG. 1, the fabrication process provides an NPN transistor having a collector that is an N-type well. The NPN transistor also includes the base and emitter in the N-type well. In FIG. 1, the example circuit includes a voltage supply 101, a transistor 102, and a transistor 104 having a larger current density than the transistor 102, thereby requiring a larger base-emitter voltage than the transistor 102 before the second transistor 104 will turn on. The transistors 102, 104 are isolated by coupling their respective collectors directly to the voltage supply 101. A resistor 109 is placed in series with the transistor 102 to measure the difference between the base-emitter voltages of the transistors 102, 104. A resistor 108 is placed in parallel with the transistor 102 and a resistor 108 having a substantially equal resistance to resistor 106 is placed in parallel with the transistor 104. The resistor 109 and the resistor 106 are coupled at node 110 and the emitter of the transistor 104 is coupled to the resistor 108 at node 115.
The nodes 110 and 115 are also the inputs of a control circuit 120, which mirrors the voltages and currents between the nodes 110, 115. In other words, the voltages at nodes 110 and 115 are substantially equal and the current flowing from nodes 110 and 115 into the control circuit 120 are also substantially equal. The transistor 104 sets the voltage at node 115 to the base-emitter voltage drop below the voltage supply 101. Therefore, the current flowing through the resistor 108 is the base-emitter junction voltage of the transistor 104 divided by the resistance of the resistor 108. As temperature increases, the base-emitter voltage decreases, thereby causing the current through resistor 108 to be the CTAT current, ICTAT. The voltage at the node 110 is forced to be the voltage of node 115, thereby forcing the CTAT current to also flow into node 110 via the resistor 106.
Additionally, because the transistors 102 and 104 have different current densities, their respective base-emitter junction voltages differ and the current flowing through the resistor 109 will be the based on the difference in the base-emitter junction voltages of the transistors 102 and 104 and the resistance value of the resistor 109. As temperature increases, the increasing difference in the base-emitter voltages of transistors 102 and 104 cause the current flowing through the resistor 109 to increase, thereby causing the voltage across the resistor 109 to increase as temperature increases. Thus, the current flowing through resistor 109 forms the PTAT current, IPTAT. The sum of the PTAT current and the CTAT current is the constant current, ICONST. In the example of FIG. 1, the CTAT current, ICTAT, and PTAT current, IPTAT, are generated in a single voltage loop.
However, to sense the PTAT voltage VPTAT, an operational amplifier 130 is coupled to the node 115. The operational amplifier 130 forces the voltage at an emitter of a transistor 140 to be the difference between the base-emitter voltage of the transistor 140 and the voltage source (i.e., VSS−VBE). In the example of FIG. 1, the transistor 140 may have the same current density as the transistor 102. Because the base and collector of the transistor 140 are coupled to the voltage source and the voltage across the base-emitter junction is forced by the operational amplifier 130, the transistor 140 is forced to source the PTAT current. To generate the PTAT voltage, a current mirror 150 may be implemented to mirror the current, thereby copying the PTAT current and forming PTAT voltage drop across the resistor 160.